This invention relates generally to a heat and pressure, color fuser for an electrophotographic printing machine, and more particularly the invention is directed to a droop compensated fuser.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent image on the photoconductive member. The toner attracted to a donor roll is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser (NFFR), the heated fuser roll is provided with a layer or layers that are deformable by a harder pressure roll when the two rolls are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll.
The heated fuser roll is usually the roll that contacts the toner images on a substrate such as plain paper. In any event, the roll contacting the toner images is usually provided with an abhesive (low surface energy) material for preventing toner offset to the fuser member. Three materials which are commonly used for such purposes are PFA, Viton.TM. and silicone rubber.
All NFFR fusers, as practiced by the industry, exhibit droop when the thermal load increases. The phenomena of droop occurs when a Nip Forming Fuser Roll (NFFR) switches from the standby mode of operation to the run mode.
Due to thermal inertia of the fuser roll core, an internal lamp cannot prevent droop. In monochromatic (i.e. one color images only) fusers where droop takes place, the effect on copy quality is not visible or noticeable to the customer. In fusing color images, the fuser roll temperature affects the appearance of the copy. Thus, the gloss and colors of color images can be adversely affected by droop. Therefore, it is important that all fusing be done at a substantially constant fuser roll temperature.
The object of this invention is to provide a NFFR color fuser wherein the phenomena of droop is minimized.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 4,567,349 granted to Henry et al on Jan. 28, 1986 discloses a heat and pressure fuser apparatus for fixing toner images to a substrate. The apparatus is characterized by the fact that silicone oil release agent material which is usually required for such devices is unnecessary. The fuser member which contacts the toner images comprises an outer layer of solid abhesive material capable of retaining this property without degradation over the operating life of the apparatus. The fuser member is so constructed that the abhesive coating contributes to the formation of the nip created between the fuser member and a backup roller.
U.S. Pat. No. 4,197,445 granted to Rabin Moser on Apr. 8, 1980 discloses a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising thermoplastic resin. The apparatus includes a heated fuser roll cooperating with a backup or pressure roll to form a nip through which the copy substrates pass at relatively high (i.e. 12-20 in./sec) speeds with the images contacting the heated roll. The heated fuser roll is characterized by a relatively thick (i.e. 10 mils or greater) outer layer or surface which by way of example is fabricated from a highly insulative material such as silicone rubber or Viton to which a low viscosity polymeric release fluid is applied. Elevating the temperature of the heated roll during a standby or warm-up is accomplished by an internally disposed heating element and the operating temperature thereof during the run mode of operation is effected by an external heater.